1. Field of the Invention
This invention relates generally to electrical switches, and more specifically to modular electrical switch assemblies in which a number of switch units are simultaneously actuated by cam members fixed to a common shaft.
2. Brief Description of Prior Art
Electromechanical valve actuators are used to open and close valves controlling fluid flow in a great variety of process environments. For example, electromechanical valve actuators are frequently found in power generating stations, oil refineries, and chemical manufacturing plants. The ability to quickly and accurately open and close fluid control valves from a central location is essential to efficient operation and safety.
Electromechanical valve actuator frequently include gear driven limit switch assemblies for valve actuator control and state sensing purposes. Typically, a geared limit switch assembly includes a mechanical counter, gear-driven directly from the actuator power train, which is used to count actuator drive sleeve turns. Drive sleeve turns (clockwise or counterclockwise) are used to predict valve stem location (e.g., open, closed, midtravel). The counter section of the geared limit switch often includes a clutch so that it can be disengaged and the counter travel may be set to correspond with distinct valve stem locations. Typically, a geared limit stitch has one or more output shafts fitted with a number of cams The cams in turn are employed to make or break electrical contacts as the output shaft rotates through predetermined positions or set points corresponding to the selected valve stem locations. The contacts are connected as elements in electric circuits which control the operation and sense the state of the valve actuator.
A key requirement for electrical contacts is that they retain excellent electrical conductivity during service event after many many make/break cycles. Contact fouling by the deposit of dirt or formation of an oxide coating on the surface of the contacts reduces their electrical conductivity. When electric contacts are opened or closed arcing between the contacts may occur. Deleterious arcing can be aggravated be fouled contacts, and can lead to pitting of the contact surface. Contact fouling is often aggravated by industrial environments. However, contact fouling can be reduced by moving the surface of one contact of a pair across the surface of the other contact after the contacts have been closed ("wiping"). Electrical switches having contact wiping action are disclosed, for example, in U.S. Pat. Nos. 4,504,713, 4,514,609 and 4,650,935.